This invention relates to an electron gun, and more particularly to an electron gun for use in cathode ray tubes.
The main lens of an electron gun utilized in a cathode ray tube is generally classified into two types, that is the bipotential type and the unipotential type. In the case of the bipotential type main lens, it is possible to form a sharply defined beam spot but as the beam current increases, the quality of the beam spot is gradually degraded. In the case of the unipotential type main lens, the quality of the beam spot is not influenced by the beam current, but the beam spot is not sharply defined. Consequently, with these types of the main lenses, if one tries to decrease the size of the beam spot, the aberration would tend to increase. For this reason, there is a limit for the sharpness of the beam spot.
It has also been proposed to construct a main lens system from a plurality of discrete lens systems so as to successively focus an electron beam by the discrete lens systems. However, as this construction requires a number of lens systems, it is necessary to lengthen the length of the electron gun.
In each of the prior art electron guns, as the electron lens system is designed such that the electron beam will pass through a region outside the near axis region of the electron lens system that is the portions remote from the optical axis of the electron lens, if one tries to obtain a high density electron beam, the aberration of the electron lens system will be increased thus making it difficult to form a sharply defined beam spot on a fluorescent screen.